Narrow Structure in the Excitation Function of $\eta$ Photoproduction off the Neutron

The photoproduction of $\eta$ mesons off nucleons bound in ${}^2\mathrm{H}$ and ${}^3\mathrm{H}\mathrm{e}$ has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the $\eta \to 2\gamma$ and $\eta \to 3{\pi }^0$ decays and the recoil nucleons were detected with an almost $4\pi$ electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of $\gamma n\to n\eta$. The results from the two measurements are consistent, taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are $W=(1670\pm 5)\mathrm{MeV}$ and $\Gamma =(30\pm 15)\mathrm{MeV}$. For the first time precise results for the angular dependence of this structure have been extracted.

Figure 1

Top: Total cross sections ${\sigma }_p$ (coincident protons, blue squares) and ${\sigma }_n$ (coincident neutrons, red circles) as a function of incident photon energy $E_{\gamma }$. Left-hand side: deuterium target. Right-hand side: helium target. Bottom: same as a function of reconstructed $\eta N$ invariant mass $W$. Black stars: results for free proton [32]. The open red circles are the present data after subtraction of the fitted $S_{11}$ and background components. Curves: fit results for $S_{11}$ resonance (dash-dotted), background (dotted), narrow structure (dashed), and full fit (solid). Insets for all figures: ${\sigma }_n/{\sigma }_p$ ratio from present work (red circles) and from Ref. [6] (black triangles).

Figure 2

Excitation functions for $\gamma n\to n\eta$ from the deuteron target. Left-hand side from $\eta \to 2\gamma$ decay, right-hand side from $\eta \to 6\gamma$ decay. Solid curve and dashed curves: fit components like in Fig. 1. Open symbols: data with background fit subtracted (scaled up by factor of 2). Curves at bottom: experimental resolution dashed (black), fitted signal (red), intrinsic signal shape (light blue) (see text).

Figure 3

Excitation functions for $\gamma n\to n\eta$ for different ranges of $\cos ({\Theta }_{\eta }^{\star })$ measured with the deuterium target. The fit curves are as in Fig. 1.